The present invention relates to linear compressors and more particularly to an improved method for sensor-less control of the linear compressors.
In the past six to seven years, linear compressors have gained increased popularity due to simplifications in their mechanical structure, ease of use when driven at both fixed and variable capacity, and higher efficiency.
Linear permanent magnet (PM) machines are very simple. They are formed from a fixed coil and a moving magnet or, vice-versa, a fixed magnet and a moving coil. Such linear PM machines are well known in the audio field as the basic voice coil actuator for loudspeakers.
The mechanical structure of the linear compressors is greatly simplified in that the piston arrangement, commonly driven by a rotating electrical machine through complex mechanical couplings, is now driven directly as a linear PM machine.
Also, the thermodynamic efficiency of the linear compressor is improved when gas leakage, existing in the piston/cylinder arrangement of the linear compressor is greatly reduced.
One of the most critical points in driving linear compressors is avoiding damaging the piston with an end of the cylinder that the piston moves into, where the discharge valve is normally placed. If this occurs, damage to the valve will occur or, long term reliability of the mechanical structure will be affected.
In compressor engineering terms, the cylinder end is called top dead point (TDC), the aim of the control is to move the piston in a way that it reaches TDC near zero, where maximum compression ratio is achieved. Another aim of the control is to control the piston movement so precisely that any distance from TDC is reached, when a compression ratio lower than the maximum is desired. This occurs, for example, when variable capacity is required of the compressor.
FIG. 1 illustrates a linear compressor structure 10. Spring 6 is added to the moving piston 1, which usually equipped with permanent magnets 4 when the fixed coil-moving magnets arrangement is implemented.
The combination of the piston mass with the springs is a mechanically resonant system, and the force required to move this system is provided by any current flowing into the coil 5, interacting with the flux generated by the permanent magnets 4 mounted on the piston.
Such current is usually an AC current with frequency that is tuned at the same mechanical resonance as that of the piston/spring arrangement. The current may be a sinusoidal or any AC waveform. The resonance curve of the mechanical part is usually so stiff that any harmonic higher than the fundamental in the coil's current does not produce any significant effect.
Unfortunately, the mass/spring arrangement also includes a very unpredictable gas spring effect. The gas spring effect is not symmetric with respect to the piston rest position, i.e., a position of the piston when a current of the coil 5 is equal to zero. In fact, the force that the gas exercises, always acts in the same direction with high values during the compression phase and low values during the suction phase.
Early attempts to accurately control piston movement utilized piston position sensors. Such sensors are bulky and expensive, moreover, getting sensor cables out from the sealed compressor's shell is hard and creates additional problems.
Recently, several sensor-less control schemes have been proposed. Some of these schemes are discussed below. Almost all of the schemes may be seen as simplification of a more general control scheme known as the Luenberger observer. Most of the sensor-less control schemes use a simplified observer model, and some external mechanical parameters, i.e., temperatures or pressures, to correct the observer's predictions as the load, e.g., a gas spring, is strongly changing with the operating conditions
In almost all of the schemes, 50 Hz or 60 Hz operation is described. The reason for this choice of the line frequency as the mechanical resonance frequency is back-compatibility with fixed speed compressors. The fixed speed compressors are running directly from the line voltage, without any electronic control. This, however, may not be the best choice in term of optimization of the mechanical structure.